In the combustion of fossil fuels in firing plants, environmentally harmful pollutant emissions are created, particularly in the form of nitrogen oxides (e.g. NO, NO2), which are usually described collectively as NOx. Pollutant emissions can be affected and/or reduced by steps taken in the design in firing plants, particularly in the burners used therein.
In the case of nitrogen oxides, the recirculation of exhaust gases created during combustion has proved to be effective. Recirculated exhaust gases lower the flame temperature so that the quantity of nitrogen oxides that are created at high combustion temperatures is reduced.
DE 195 09 219 discloses a method and a burner head in which fuel gas is combusted while combustion air is supplied, with inert gas to reduce nitrogen oxides, where the fuel gas is blown in relative to the combustion air on two levels lying one behind the other in the direction of airflow, with an overstoichiometric combustion air/fuel gas mixture on the first level upstream from the flame at the flame root, and with supplemental fuel gas supply in the second plane, and where recirculated exhaust gas serves as inert gas which is supplied on the second level, and one part of the combustion gases blown in on the second level forms an understoichiometric mixture with the recirculating exhaust gas before the mixture is brought to the flame.
EP 0 347 834 discloses a burner head for a forced-draft gas burner having a burner tube containing a device for distributing the fuel gas and the combustion air and also fuel gas nozzles and air passages and having a flame tube adjoining the burner tube, where at least one radial opening for exhaust gas recirculation is present between burner tube and flame tube, and, in the root area of the flame between burner tube and flame tube upstream from the radial opening but downstream from the fuel gas nozzles and air passages, webs are provided projecting radially inward transverse to the burner head longitudinal axis.
EP 0 635 676 discloses a method for the low-NOx combustion of liquid or gaseous fuels in firing plants with a burner projecting into a combustion chamber of a boiler, whose burner tube has at least one fuel nozzle located therein to supply the fuel and an adjacent baffle plate, in which method a considerable portion of the fuel is supplied to an area downstream from the baffle plate and adjacent to the inner wall of the burner tube, exhaust gases present in the combustion chamber are brought back through internal recirculation into negative pressure areas in the burner tube which build up downstream from the baffle plate and one or more guide elements pierce the burner tube and project into the negative pressure areas.
EP 0 857 915 discloses a method for combusting liquid and/or gaseous fuels with a combustion head of a burner projecting into the combustion chamber of a boiler, the flame tube of the burner having at least one atomizer nozzle located therein for the supply of fuel and a swirl body, where a portion of the air stream for combustion is led through a swirl body, negative pressure is generated at the section of the flame tube located downstream by guide devices and openings in the flame tube, the exhaust gas from combustion is mixed with the combustion air, the exhaust gas is mixed with the atomized but not yet ignited fuel, and a flame front is generated at a distance from the combustion head.
Overall, it can be said that known approaches are not adequate to meet the increased requirements for emissions resulting from the operation of firing plants, in particular to achieve the low nitrogen oxide limits which are required, for example, by law.